Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protection Agency (EPA) has promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hydrocarbon and nitrogen oxide pollutants, it has become the practice to employ catalysts in conjunction with air-to-fuel ratio control means which function in response to a feedback signal from an oxygen sensor in the engine exhaust system. Although these three component control catalysts work quite well after they have reached operating temperature of about 300.degree. C., at lower temperatures they are not able to convert substantial amounts of the pollutants. What this means is that when an engine and in particular an automobile engine is started up, the three component control catalyst is not able to convert the hydrocarbons and other pollutants to innocuous compounds. Despite this limitation, current state of the art catalysts are able to meet the current emission standards. However, California has recently set new hydrocarbon standards (similar standards have been promulgated nationwide) which can not be met with the current state of the art three component control catalysts.
The art contains several references which describe systems that have been devised to attempt to solve this cold start emission problem. One such reference is U.S. Pat. No. 3,699,683 in which an adsorbent bed is placed after both a reducing catalyst and an oxidizing catalyst. The patentees disclose that when the exhaust gas stream is below 200.degree. C. the gas stream is flowed through the reducing catalyst then through the oxidizing catalyst and finally through the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. When the temperature goes above 200.degree. C. the gas stream which is discharged from the oxidation catalyst is divided into a major and minor portion, the major portion being discharged directly into the atmosphere and the minor portion passing through the adsorbent bed whereby unburned hydrocarbon is desorbed and then flowing the resulting minor portion of this exhaust stream containing the desorbed unburned hydrocarbons into the engine where they are burned.
Another reference is U.S. Pat. No. 2,942,932 which teaches a process for oxidizing carbon monoxide and hydrocarbons which are contained in exhaust gas streams. The process disclosed in this patent consists of flowing an exhaust stream which is below 800.degree. F. into an adsorption zone which adsorbs the carbon monoxide and hydrocarbons and then passing the resultant stream from this adsorption zone into an oxidation zone. When the temperature of the exhaust gas stream reaches about 800.degree. F. the exhaust stream is no longer passed through the adsorption zone but is passed directly to the oxidation zone with the addition of excess air.
Canadian Patent No. 1,205,980 discloses a method of reducing exhaust emissions from an alcohol fueled automotive vehicle. This method consists of directing the cool engine startup exhaust gas through a bed of zeolite particles and then over an oxidation catalyst and then the gas is discharged to the atmosphere. As the exhaust gas stream warms up it is continuously passed over the adsorption bed and then over the oxidation bed.
U.S. Pat. No. 5,051,244 discloses a process where an adsorbent is placed in front of a catalyst. Initially the exhaust is flowed through the adsorbent bed and then through the catalyst. After the adsorbent bed is at a temperature of 150.degree. C., the exhaust flow is diverted around the adsorbent bed and through the catalyst. Once the catalyst has reached operating conditions, the hydrocarbons which were adsorbed on the adsorbent bed are desorbed by flowing exhaust gas through the adsorbent and then bypassing the adsorbent bed.
German Patent Application No. 2,214,772 discloses a process in which the exhaust is flowed first through a reducing catalyst, then through a charcoal filter and then through an oxidation catalyst. After the system has reached its operating temperature, the charcoal filter is bypassed in order to prevent oxidation of the charcoal.
U.S. Pat. No. 4,985,210 discloses a process for treating an automotive exhaust by using an adsorbent bed followed by a catalyst. The adsorbent bed contains a mordenite or a Y-type zeolite.
U.S. Pat. No. 5,125,231 discloses a system comprising a catalyst followed by a catalyzed adsorber. Engine exhaust is selectively conveyed to the catalyst or catalyzed adsorber such that hydrocarbons are first held by the adsorber and then released and recycled through the catalyst when the catalyst has reached its operating temperature.
Finally, U.S. Pat. No. 5,140,811 discloses a process in which an adsorber is placed in front of a catalyst. When the gas temperature reaches 200.degree. C., the adsorber is bypassed and when the gas temperature reaches 400.degree. C., the gas flow is again passed through the adsorber and then through the catalyst.
In contrast to this art, applicants have developed a unique system in which the catalyst and molecular sieve bed (adsorber) are arranged side by side with a connecting pipe between and parallel to them. When the engine is first started, the relatively cool engine exhaust stream is diverted through the catalyst bed, then through the connecting pipe, then through the molecular sieve bed and finally discharged to the atmosphere. The molecular sieve bed will preferentially adsorb hydrocarbons versus water under the conditions present in the exhaust stream.
As the engine exhaust stream warms up, both the catalyst bed and the molecular sieve bed will also warm up. At a temperature of about 150.degree. C. to about 200.degree. C. the hydrocarbons on the molecular sieve bed will begin to desorb. Before the hydrocarbons begin to desorb from the molecular sieve, the engine exhaust stream is diverted such that it is first flowed through the molecular sieve bed, then through the connecting pipe, then through the catalyst bed and finally discharged to the atmosphere. Finally, when all the hydrocarbons have been desorbed from the molecular sieve bed, the engine exhaust stream is again diverted so that it flows through the catalyst bed and then discharged to the atmosphere.
None of the art cited hints at a process where the engine exhaust stream is first flowed through a catalyst and then an adsorber but at a later time the exhaust stream is first flowed through an adsorber and then through a catalyst. Applicants' invention affords a very compact and efficient pollutant control system and process which is completely different from the various processes and systems found in the art.